In an existing Long Term Evolution (LTE) frequency division duplexing (FDD) system, a terminal device performs channel estimation by using a reference signal sent by a network device and then determines state information of a downlink channel. The terminal device feeds back the state information of the downlink channel to the network device. For example, the terminal device sends a precoding matrix index (PMI), a rank index (RI), and a channel quality index (CQI) to the network device. The network device selects a precoding matrix based on the index information. The network device performs processing by using the precoding matrix, to improve downlink communication quality. Therefore, accuracy of the channel state information fed back by the terminal device affects downlink communication quality. How to improve the accuracy of the channel state information fed back by the terminal device is a problem that needs to be resolved.